The invention of the transistor enabled small, portable, and reliable audio devices. As a result, people moved from having plug-in home radios, to having small transistor radios, boom boxes, portable tape players with headphones, and now to the ubiquitous use of portable MP3 players by people of all ages. People also listen to the radio or recorded music in their cars, and through headphones while commuting, working, or walking around town. Such devices also are used to produce audio streams other than music or radio, including audio books, telephone conversations, navigational instructions (e.g., “turn right”), warnings (“please put on your seat belt”), and confirmations (“you are listening to 101.3 FM”).
With all of these available audio sources, and with many of these sources being combined by a single unit, such as by a smart telephone or vehicle information system (built into the vehicle's audio system), it is not surprising that a user will be faced with multiple incoming audio streams at once. In some systems, when such an event occurs, a most important audio stream may be determined (e.g., warnings are more important than telephone calls, which are more important than music or radio), and that most important stream may be played while other sources are muted or substantially muted. When the most important stream ends, the other streams may be brought back in amplitude until another important audio stream is received by the user's device.